In an optical transmission system, it is desired to rapidly deal with failure on a transmission path and a failure in a device therein. Patent Literature 1 discloses a technique of dealing with failure on a transmission path or a failure in a device in an optical transmission system.
In an optical transmission system, an LOS (Loss Of Signal) signal may be used as not only for an alarm about a state of a network but also a trigger that switches the transmission path to a redundant path. In this case, a photo detector or FEC (Forward Error Correction) processor disposed in the receiver may monitor loss or deterioration of an optical signal.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2003-134074
In an optical transmission system, detection of an LOS signal in a short time is very important. An LOS state is detected unless an optical signal reaches the receiver in the event of occurrence of a problem with the optical signal at an optical transmitter or the transmission path. However, an input level of an optical signal may not decline due to superimposing an ASE (Amplified Spontaneous Emission) light onto the optical signal in the transmission path.
In this case, it may be difficult to detect an LOS state. This means that it takes time to detect an LOS state, which may interfere with a route switching in a predetermined time period. In particular, the number of transmission spans has increased in accordance with recent improvement in the performance of the receiver, and consequently, the ASE light power in an LOS state is large. For the above, it is difficult to detect an LOS state through the use of light power.
Alternatively, an LOS state may be detected through the use of a BER (Bit Error Rate) or the number of error detections by FEC (Forward Error Corrections). However, detection based on a BER or the number of error detections by FEC takes time, so that these pieces of information are not suitable for signals for switching the route.